NEUTRON POWDER DIFFRACTION STUDY OF DEUTERIUM ORDERING IN BETA-PHASE PD-D AT LOW-TEMPERATURES

We recently briefly reported the first observation by powder diffracti on of a superlattice in beta-Pd-D annealed at low temperatures. In thi s paper we give more details of our investigation by neutron powder di ffraction of the low temperature phase diagram at several concentratio ns and temperatures. Two different superlattice were observed, at (1 ( 2n + 1)/2 0) positions in PdD0.62 and PdD0.65 annealed at 54 K, and at (h +/- 2/5 h +/- 4/5 0) and equivalent positions in PdD0.77 annealed at 70 K. At the lower concentrations, contrary to the previous reports of a tetragonal 14(i)/amd structure, a cubic superstructure in <P(m)o ver bar 3n> with a doubled cell constant relative to the disordered st ructure was determined. The diffusion of D in 6d sites into neighbouri ng 6c sites generates the order, driven by a repulsive nearest-neighbo ur interaction. The ordering at [D]/[M] = 0.77 developed very slowly a t 70 K and is described in an MoNi4 structure as in previous observati ons. No sign of ordering was found in PdD0.82 and PdD0.86 at temperatu res down to 20 K. Enhanced anisotropic line broadening during ordering was observed in the Pm3n structure. It was successfully accounted for in the refinements of the cubic superlattice by assuming that it orig inates from strain along [001] which propagates anisotropically in oth er directions.